Do you have an ISP that has dialup access? I ask this because a friend asked me how I connected over the phone lines after he tried, but he didn't have a dialup access account.
If you have such an account then using a program like wvdial works for me. It comes with its own instructions.
In Gnome you can also use system-config-network (I use fedora, I hope mint uses the same app). If Mint recognizes the modem, you should be able to configure it from there, UNLESS, it is a dreaded winModem, then you have to go through some more configuration steps.
If you can do:
lspci and lsusb from the terminal and post the output here, we may be able to help more.

I have a Palm Treo 700p, in order to tether it to the computer I had to add an app called USBModem. When USBModem is activated on my cell phone, I can use a program like wvdial to call into verizon and connect to the internet. The standard phone number that is used (if I remember correctly) is 99*, although you might have to get an access code from your cell phone provider in order to log on.
With the number of Wifi locations growing as they have, I haven't had to use this method to access the internet in a while. I've also changed distros since then, so the information I had stored is gone. I've been meaning to try it again, and I intend to this week sometime. Once I get it running again, I'll let you know the steps I took.
The Treo 700p uses a palm operating system (I never wanted a phone with windows mobile), and, the USBModem app was available for that system. Why don't you try googling "your cell Phone+usb+modem" and see if you come up with anything for your particular phone. You might want to add "debian" to your search since Mint is based on Debian and see if that helps.

p.s. I tried googling and found this in your phones manual: (I googled "motorola+i335+boost+usb+modem")
When used as a modem, your handset is
enabled for Packet Data transfer mode, which is
used for small file transfers such as email.
Note: To use these services, you must install
the iDEN Wireless Data Services software
(available separately). For more information on
setting up your computer and your handset
for packet and circuit data calls, contact your
service provider.

Verizon has a Desktop program that I can use with windows. I hope that iDEN Wireless Data Services software doesn't have that requirement. If it does, try googling as I described above and see if anyone has any other apps that can work. My USBModem app works apart from the Verizon Desktop, so, I can use wvdial with it to bypass the proprietary programs.
BTW, there were many more selections to look at after googling, take a look, maybe one of them has your solution. I'm not familiar with Boost Mobile, so, I can't help you with the services they provide.

I hoped this helped a little.

LOL...back again, check out this Yahoo answers link, same question, better answer (and links).
http://answers.yahoo.com/question/index?qid=20090327184108AACJ0e6

OK, so that app was similar to my Verizon Desktop software. What about the USBModem app that allows me to use my Palm Treo like a modem with Linux. Did you look in your app store or search for that? It's been a few years since I've searched, but an app like that might be your only hope of connecting Linux to the internet through your phone.

Sorry I can't be more help.